The invention relates to a process and an apparatus of the type wherein a gas or vapor is used to react with or treat a particulate type of contact material such as a catalyst, which is passing by gravity from one reaction or treating zone to another. Examples of processes carried out in such an apparatus include various hydroprocessing techniques such as catalytic reforming, catalyst regeneration, hydrotreating, dehydrogenation of butane and dehydrocyclodimerization, to name a few. A specific example of a catalyst regeneration apparatus in which a catalyst whose surface has been covered with coke during a catalytic reforming operation, moves downward through a carbon burn-off section, through a halogenation section and into a drying section is shown and described in Greenwood et al, U.S. Pat. No. 3,652,231, the subject matter of which is herein incorporated by reference. In this particular apparatus, the catalyst moves downwardly in the annular space between a pair of spaced concentric perforated screens and is subjected initially to a first radially flowing recycle flue gas having a relatively low oxygen content and secondly, to a radially flowing second gas containing air, a halogen and steam. The first and second gases are each brought into an annular, elongated plenum space surrounding the annular screens and are segregated within the plenum by an annular baffle ring which divides the plenum into two regions. Obviously, this arrangement permits some of the recycle gas to diffuse downwardly into the catalyst region being subjected to the second gas while permitting some of the second gas to diffuse upwardly into the catalyst region being subjected to the first gas. To the extent there is diffusion, it is difficult, if not impossible, to determine if there is any particular location in the annular space in which the catalyst could be sampled so as to provide a sample which could be said to be truly representative of the catalyst after its coked surface had been burned off to a desired extent. In usual practice, although sampling devices such as those shown in U.S. Pat. Nos. 3,786,682 and 3,973,440 do exist, it is common, when attempting to measure the extent to which coke has been burned off, to operate only the burn-off zone of the regenerator without operating lower zones such as halogenation and drying. Then, after the regenerator has been operated for a sufficient number of hours to permit catalyst to pass through the inoperative halogenation and drying zones and to get to the bottom of the vessel, a sample is taken of the catalyst leaving the vessel. Such a method obviously is very time-consuming and is thus sometimes done only upon startup of operation.